Bolt Patterns

Do you know which alternator you have or will need? Below you see in our guide that there are five different alternator bolt patterns that came on LS engines. These are LS truck (most common), Corvette, CTS-V (also known as LSA vehicles), LS4, and LS9. The LS4 and LS9 alternators both have unique bolt patterns that only interchange with their specific engine RPO code. They have a distinct side bolt pattern.

Next up we have the LSA powered vehicles such as CTS-V and ZL1 Camaros, and also select Australian variants. These have the widest bolt pattern of 6.27 inches from bolt center to bolt center. The second most common is the Corvette bolt pattern, which is 6.06 inches. These were also found on 2010 to 2015 Camaros with LS3 and L99 engines. Lastly, we have the "Truck" bolt pattern alternators with 5.46 inch bolt centers. These are by far the most common bolt pattern you will find on LS engines, due to the millions of trucks and SUVs that GM produced. Since the majority of the engines used "Truck" bolt pattern alternators, that means most of our ICT Billet brackets were made to fit this bolt pattern.

Amperages: Factory vs. Aftermarket

The aftermarket has lots of great options such as billet cases and amperages up to 400 amps. If your application does not have a lot of space or you want that clean look, then grab a 105 amp case alternator. These are small enough they can easily fit in front of the passenger side cylinder head. Note that the driver's side head is too far forward to mount any alternator in front of the head and still retain the factory truck belt spacing. The aftermarket brackets you see for this such as the Holley front runner move the harmonic balancer forward a couple inches.

I know what you're thinking. "I need more than 105 amps." We have options from PowerMaster and Mechman that use the 105 amp case but have better internals so they actually put out amperages up to 165 amps and 240 amps respectively.

How To Calculate Amps Needed

Below is the most common items needed when running a fuel injection setup which equal about 110 amps:

• EFI Ignition System – 20 amps
• Headlights/Tail Lights – 15 amps
• Gauges – 5 amps
• Radio – 10 amps
• HVAC – 20 amps
• Electric Fuel Pump (each) – 10 amps
• Electric Fan (each) – 30 amps

Next are common items we see added to builds. Pair your total size with an alternator that is rated for that amperage:

• Transmission Fan – 25 amps
• Intercooler Pump – 15 amps
• Electric Water Pump – 20 amps
• Datalogger – 5 amps
• Transbrake – 15 amps
• Nitrous Solenoids (each) – 15 amps
• Line Lock – 6 amps
• Scavenge Pump – 15 amps
• Electric Power Steering – 15 amps

Common Alternator Amperages We Stock

Here are the common alternator amperages that we stock:

Wire Connector Styles and Why This Matters

A big consideration here is the wire connector style if you don't want to modify your wiring. There are four connection types to be aware of: 4 pin (early years) wire connector, Mitsubishi 2 pin (GTO only), 2 pin (late model cars), then the aftermarket "one" wire alternators. The one wire only uses the large gauge wire on the rear output stud of the back, which is directly connected to the battery.

If you are sticking to the same connector style, let's see how to find these at your local parts store. 4 pin connector alternators were found on early year LS vehicles such as 2000 Silverado. A good rule of thumb is the smaller the engine size, this typically follows the option level of the vehicle, which dictates the alternator amperage. For example a Silverado with 4.8L and low trim level (no power windows) would have a 105 amp alternator, versus an Escalade ESV with 6.0L would have a 160 amp, which has an old style 4 pin connector.

These used a simple on/off 5 volt power source from the ECM. If this is inoperable then use our pigtail with built in resistor (WPALT31) connected to a switched 5 amp, 12 volt power source. The exception to this is that all Mechman alternators require a 12 volt (not 5 volt) switched power source.

Case Sizing is Important

The most common alternators are found on truck engines. The early LS powered trucks used a 105 amp alternator. This alternator has the smallest case, which allows it to fit in tighter locations for the cleanest look on your swap. We recommend this case size every time unless you need a higher amperage than this case size can support.

Next up is the 145 amp case. See the below chart for how the various case sizes affect the depth and diameter of the case size. Since the 145 amp case alternators are larger diameter, this moves the pulley further away from the bolt holes, which increases belt length. Belt length will vary depending on how much wrap the belt has on the pulley. For example if the belt is running tight around the pulley in a U shape then you would roughly double the length of how far the pulley moved up. If your belt wrap was more of an L shape then the length will drop.

Next we have the largest case that GM offered. These were offered as 160 amp, which has the same pulley location as the 145 amp case alternators. This 160a case is too deep to fit in front of the passenger side cylinder head since it is deeper. The good news is that the highest amperage alternator that Mechman offers is a 400 amp, with a 145 amp sized case for those of you needing a lot of power for a sound system.

Dual Alternators

We do also offer dual alternator brackets such as 551777-3DA or can mount a second alternator 551780-3 in a different location on your engine to double output. We offer dual alternator harness options such as WAALT31-28DBL (4 pin) and WAALT41-36DBL (2 pin) to make it plug in play.

Decoupler Pulleys

GM used decoupler pulleys on select high-end LS vehicles to reduce belt noise and protect the accessory drive system. These pulleys, also known as one-way clutches or overrunning alternator pulleys (OAP), allow the alternator to freewheel in one direction when engine RPM drops suddenly. This decoupling prevents the alternator’s heavy internal rotor from putting stress on the serpentine belt during quick deceleration.

This matters for a few reasons. On high-performance engines that rev up and down quickly, the alternator’s inertia can cause belt slap or loosen the belt when RPMs drop rapidly. The decoupler pulley eliminates that problem by letting the alternator spin freely instead of forcing the belt to absorb that load.

Decoupler pulleys also help smooth out vibrations. In a normal setup, the crankshaft’s power pulses are transferred into the belt drive, which can cause noise and wear on the tensioner. The one-way clutch absorbs those pulses, leading to quieter, smoother belt operation and longer belt life.

Another benefit is efficiency. When the alternator can overrun during deceleration, the engine doesn’t have to slow down as much rotational mass. That means less drag on the engine and slightly better fuel economy, especially in stop-and-go driving.

Overall, these pulleys reduce belt wear, lower vibration, prevent belt chirp or slippage, and extend the lifespan of both the alternator bearings and the tensioner. GM mainly used them on higher-end or performance LS models where rapid RPM changes are common.